This invention relates to a method of making electrostatographic images, and more particularly to an electrostatographic method of producing high quality, high resolution images.
In the art of electrostatography, latent electrostatic images are formed on a surface. Thereafter the latent images are rendered visible by contact with an electrostatic developer composition. Generally, two different types of developer compositions have evolved on the commercial scene. These are classified as dry developers and liquid developers. Dry developers include electroscopic marking particles called toner particles which are employed with or without separate particles to form two component developers or single component developers, respectively. Liquid developers employ a carrier liquid together with marking particles.
Each of these development techniques, have found widespread use in the marketplace. Also, each has disadvantages which require different approaches when viewed from a commercially acceptable perspective. Inherently, liquid development systems are capable of higher quality reproduction of the original image because the particle size of the electroscopic marking particles (toner) are much smaller than that employed in dry developers. Liquid developers transfer readily from the dielectric layer or photoreceptor to the receiving sheet because the transfer takes place while the toner particles are still wet with the carrier liquid.
Dry development systems, on the other hand, are limited with respect to the copy quality of the final image on the receiver sheet by the size of the toner particles. U.S. Pat. No. 4,284,701 issued Aug. 18, 1981 speaks of this in these terms, at Col. 1, line 58 et sec. "Copy quality includes such things as image clarity, i.e., clear delineation of lines; uniform darkness of image areas; background quality, i.e., grayness or lack of it in the background areas; and other somewhat intangible features that go toward making a good `quality` copy."
Various techniques have been suggested to improve the copy quality of the electrostatographic images including that taught and claimed in the above-mentioned patent which accomplishes this to a certain extent by rigidly controlling the size of the toner particles by a classification technique. U.S. Pat. No. 3,969,251 issued July 13, 1976 also employs classified toner particles. European Patent specification No. 0,010,375 utilizes the classified toner particles of previously mentioned U.S. Pat. No. 3,969,251 together with a dual transfer apparatus. The large particles are transferred at a differnt station than that employed for transferring the smaller particles. In these references, as well as in the commercially available electrostatographic copy devices the predominant toner particles have a volume average size of 8 to 12 microns, but generally include particles having much larger and smaller particles.
Thus, in dry development systems, the resolution of the final image is limited by the particle size of the toner employed and the lower limit of particle size is limited by the forces present on the particles which control whether or not a transfer will occur efficiently. The efficiency drops off as the particle size decreases and more toner remains behind on the photoreceptor. Moreover, the residual toner is more difficult to remove. Both of these effects escalate cleaning problems. The photoreceptor must be clean of toner particles for the start of the next immediate imaging process. Thus, the transferability of the developed toner image is the limiting factor with regard to the quality of the completed image with respect to resolution.
In order to obtain maximum image clarity of transferred images (as quantified by granularity measurement or other parameters which relate to image resolution), it is important to maintain as low a mean particle size for toners as possible. If the transferred toner particles are too large, fine detail in an image cannot be satisfactorily resolved. The granularity of the completed image tends to increase with the toner size. However, it is found that fundamental difficulties arise when trying to transfer toner particles having an average radius less than 5 .mu.m. This difficulty in transferring small particles is referred to in "Xerography And Related Processes" by J. H. Dessauer and H. E. Clark, editors, published by the Focal Press, London and New York 1965 at pages 393 and 394, an article by N. S. Goel and P. R. Spencer entitled "Toner Particle-Photoreceptor Adhesion" published in Polymer Science Technology, 1975, 9B, page 821 and also an article entitled "Forces Involved in Cleaning of an Electrophotographic Layer" by L. Nebenzahl et al (IBM) Photographic Science & Engineering 24, 293-298 (1980) which refers to IBM toner at 10 .mu.m being held by Van der Waals' forces.